Wide-band instruments in the 5G Era

by David Hall, Principal Product Manager, RF and Wireless Communications, National Instruments, Corp.

Just a decade ago, signal analyzers with 20 MHz of instantaneous bandwidth were considered state-of-the-art. Within a decade, signal analyzers with no less than 2 GHz of bandwidth will likely be entry-level devices.

Clearly there’s a trend toward signal analyzers supporting wider instantaneous bandwidth. The trend is primarily driven by advances in off-the-shelf, analog-to-digital converter (ADC) technology and wireless standards. But the benefits of faster ADCs reach far beyond the wireless industry. Improvements in off-the-shelf ADCs now let test equipment makers address the needs of a broad spectrum of industries, but particularly those of aerospace and defense.

One can understand how wireless communications have helped drive signal analyzer technology by reviewing the rapid rise in channel bandwidth across today’s modern wireless standards. For example, an AMPS communication channel (1G cellular) consumes around 30 kHz of bandwidth for one-way communication (60 kHz for full duplex), a GSM channel (2G) consumes 200 kHz, and a UMTS channel (3G) consumes 5 MHz.

The widespread development of 802.11ac devices has also had an impact. This WiFi networking standard defines high-throughput wireless local area networks (WLANs) on the 5 GHz band. It has expected multi-station WLAN throughput of at least 1 Gbit/ sec and a single link throughput of at least 500 Mbit/sec. This is accomplished by, among other things, a wider RF bandwidth (up to 160 MHz). A few years ago, this standard was ahead of the capabilities available in RF signal generators and analyzers. As a result, many test and measurement vendors accelerated their development of wider bandwidth instruments just to support the bandwidth requirements of 802.11ac in a timely manner.

Looking ahead, the next major milestone for RF test equipment is the ability to test fifth-generation cellular devices. Instrumentation makers will also respond to researchers’ use of advanced software-defined radio tools to actively prototype 5G-candidate technologies, such as massive MIMO (multiple input, multiple output), GFDM (generalized frequency division multiplexing) and millimeter wave communications. The potential use of wideband millimeter-wave signals most likely will need RF test equipment able to offer 2 GHz of bandwidth by 2017 or 2018 to support a 2020 deployment.

By any stand ard, 2 GHz of instantaneous bandwidth in an RF signal analyzer would be a major landmark. If such an instrument existed, it would be an incredibly useful tool for bandwidth-hungry applications, such as radar-pulse measurements and spectrum monitoring.

Moore’s law is one reason why the industry is going to get to 2 GHz of bandwidth. The Law, of course, is an observation that transistor density on an integrated circuit doubles every two years. But CPUs and FPGAs are not the only technologies that have benefited from exponential improvements in IC transistor density. ADC sample rates are following a similar trend. Consider the maximum available sample rate of 12-bit ADC technology over time. Ever-faster 12-bit ADCs boost the dynamic range available for analyzing frequency domain signals, so they are an effective proxy for the bandwidth capabilities of RF signal analyzers.

It is interesting to note the technical progress that firms making ADCs foresee. For example, officials at Analog Devices Inc., a company that produces about half of all ADCs sold commercially, have said they can see 12-bit ADCs pushing to 10 GS/sec over the next two to three years, and 14-bit ADCs pushing to 2.5 GS/sec in that same time frame. They also said 14 to 16 bits at 10 GS/sec is on the horizon, though they feel technical breakthroughs would be needed for these converters to come to fruition.

In 1975, a 12-bit ADC with 2-μsec settling time (approximately 500 kS/sec, though not an exact corollary) was considered state of the art. Today, the fastest sampling 12-bit ADCs are hitting rates exceeding 2 GS/sec—a feat that’s powering some of the widest bandwidth signal analyzers in the industry.

Based on the current rate of development, 12-bit converter technology will soon be able to give RF instruments instantaneous bandwidth in the multigigahertz range and boost today’s gigahertz-bandwidth oscilloscopes to even higher resolutions.

Coming to wireless instrumentation
Engineers will soon be using exciting new measurement approaches and techniques ushered in by next-generation RF signal analyzers (and even oscilloscopes). In radar design and development, for instance, instruments with better bandwidth and signal-processing capabilities should soon make it possible to design more advanced radar prototypes. In high-volume manufacturing tests, instruments will be able to acquire ultra-wideband signals in a single shot. This should help test engineers easily capture data from multiple wireless devices in parallel for faster multi-site test configurations.

In many respects, the bandwidth limitations of yesterday’s RF signal analyzers drive some of the test techniques we use today. Now that we’re in the middle of a bandwidth revolution, we need to consider how wider bandwidth will empower the test techniques of tomorrow.

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